Recently, principal population centers on the North American continent and elsewhere have experienced such severe shortages of natural gas, that industrial performance has been reduced, educational institutions have been closed under severe cold weather conditions and general business entities have witnessed a lowering in employee efficiency and performance under emergency gas conservation operating conditions. For several years, the home construction industry has been required to provide electrical energy to the exclusion of gas derived energy, thus imposing significantly higher living costs on the purchasers of new homes. While exploration for new sources of natural gas can be encouraged through financial incentives and the like, expectations of major finds at sites available to the Western world are not enthusiastic. Consequently, secondary gas resources known to be available have been given considerable attention.
Those concerned with the development of additional natural gas sources have not lost sight of the potential for converting available carbon resources to a product gas suitable as a natural gas supplement or substitute. Coal gasification long has been the subject of study. Of more current interest, however, are efforts to convert the ever increasing volumes of solid wastes of municipalities and industries to a usable gas. The direct application of coal gasification processes to lowbulk density, fibrous, low-ash feed materials like solid waste, manure and other forms of biomass is not likely to be successful because of the vast difference in physical properties between coal and these other cellulosic feed stocks. For example, most coal gasification processes employ fluid beds in which coal char is the fluidized solid. However, because of the very low bulk density of solid waste and biomass type materials and in view of their fibrous nature, they will tend to excessively elutriate from any reactor of a conversion system unless very low superficial gas velocities are used in conjunction with the systems. Low gas velocities require low throughput rates which, in turn, result in increased capital requirements for gasifiers and like components of any conversion system. For example, in the production of an intermediate - Btu gas from the low-sulfur feeds where H.sub.2 S removal is not necessary, the gasifier of such system is a large fraction of the total capital investment and thus the economics of any conversion system for these fibrous materials are greatly affected by variations in reactor throughput rates. While organic materials, per se, may range broadly from diamonds to common garbage, the types of materials contemplated for the instant conversion systems are those generally falling within the low bulk density fibrous material category principally including solid waste and biomass. Solid waste will include such materials as manure and municipal waste, while biomass is considered generally to encompass such materials as bagasse, energy crops, seaweed including kelp, cornstalks, forest residue and general plant residues.
A variety of technical approaches have been proposed in attempting the conversion of solid waste and biomass to product gas. For example a synthetic natural gas may be developed from solid municipal waste by controlled biodegradation. However, facilities having an extremely large vessel capacity operating on long solid waste residence intervals are required for carrying out such techniques. Additionally, the by-product from such systems may not be desirable and may represent a disposal problem in and of itself. Further, inorganic constituents of the waste material generally are required to be removed before the waste is introduced to the digestive process.
Another conversion technique, described, for example, in U.S. Pat. Nos. 3,729,298 and 3,817,724, seeks to develop a product gas from solid waste by pyrolysis, a system wherein the hydrocarbon solids of the waste material are subjected to relatively high temperatures to generate a methane-containing gas, as well as a relatively high quantity of tar and char.
Certain disadvantages accrue with the use of the pyrolysis procedure, the more apparent being the disposal problem for the residue and another residing in a requirement for developing the heat or thermal energy to create the pyrolysis reaction to generate product gas. For this, generally, about twenty percent of the product gas itself is drawn off from the process to generate the heat energy required. A similar approach is provided in U.S. Pat. No. 3,874,116 in which heat is supplied to the zone producing combustible gases through the burning of a portion of recycled synthesis gas. For either approach, the thermal energy demand is significant, temperatures in the range of 1700.degree. F. and up being required to be generated within the reactor. As another aspect of these systems, at such higher temperatures, should the developed product methane enter the gasification zone of the reactor, it will tend to combine with water present as steam to break down to carbon monoxide and hydrogen gas, thus leading to further losses in output efficiency. Higher temperatures pose another requirement to the systems in that the inorganic components of the solid waste material, i.e. aluminum, glass, steel and other products should be removed prior to the introduction of waste to the reactor. This follows, inasmuch as such materials have an important recovery value in and of themselves and, if subjected to the higher temperatures of the pyrolysis reaction, will tend to break down to less desirable forms, as well as represent substances using up volume while remaining inert within the chemical process.
Another conversion technique to which the instant invention is particularly addressed, involves a process conventionally referred to as hydrogasification. Generally, the hydrogasification reaction is one wherein the carbon component of the waste material is reacted with hydrogen-containing synthesis gas to produce methane. The temperature at which this reaction occurs is one relatively lower, for example, than that required for the gasification reaction, hydrogen gas generally being introduced to the reaction at about 1000.degree. F. As described in U.S. Pat. Nos. 3,733,187 and 4,005,994, the hydrogasification process is one wherein solid waste refuse is shredded and introduced into a confined pressurized zone which is generally elongate in nature and vertically oriented. As the waste material is introduced at the top of the zone, a hot, hydrogen-containing synthesis gas is introduced at the lowermost regions thereof. As the waste material migrates under gravitational force downwardly through the zone, this moisture content thereof is removed and upon complete removal of the moisture, the methane-producing hydrogasification reaction occurs and the organic material subsequently becomes a carbon containing char. This char then is moved to a gasification reactor at which location it is substantially entirely converted to hot synthesis gas in the presence of oxygen and steam and, by virtue of the exothermic nature of that reaction, thermal energy is evolved at the levels required in the hydrogasification zone. Advantageously, only a minor amount of residue requiring disposal is developed as a by-product of the synthesis gas production process. As described in the noted U.S. Pat. No. 4,005,994, a highly efficient utilization of thermal energy with the process is availed. Further, a subsidiary advantage ensues with the hydrogasification process due to the relatively lower temperatures developed within the reaction zone of the hydrogasifier. With the system, both inorganic as well as organic waste components may be introduced into the reactor. These inorganic components pass by gravity through the reactor and are subjected to temperatures which advantageously provide for their sterilization while being of such lower level as to prevent their destruction as by the sintering of glass or oxidation breakdown of metals. At the lowermost region of the hydrogasification reactor zone, the inorganic material readily may be recovered as a valuable by-product, thus enhancing the economic feasibility of this form of product gas production. While the theory and lesser scale demonstration of all of the above synthetic gas production systems has been demonstrated, practical implementation thereof accommodating those volumes of municipal waste required to be treated has not been effected without difficulty. A pyrolysis system installed in Baltimore, Maryland met with severe operational difficulties due to a variety of practical imperfections, for example as associated with the pretreatment, movement and storage of waste raw material as well as with the reactor related manipulation thereof.
Because the gas production systems should be located near their source of raw material as well as near gas distribution networks, i.e. near major population centers, they must retain a capability for accepting large volumes of waste and accommodating these volumes without creating odor and pollution nuisances. Necessary storage of the material should be of so short a residence interval as to be without significant odor nuisance. Further, the production of unwanted, polluting by-products such as tars and the like should be minimized.
In view of the significant capital expenditure represented with any given gas production installation, the necessary overall size of the facility must remain as practical in scope as possible. For further practical necessity, gas impurities, i.e. volatile constitutents which necessarily may be generated in conjunction with desired methane production must be minimized and the final synthetic gas product both should be compatible with natural gas supplies as well as must be produced evidencing a relatively constant chemical makeup or consistency.